Looper is a clever, entertaining science fiction thriller that neatly blurs the line between suicide and murder. An existential conundrum wrapped in a narrowly conceived yarn about victims sent back in time to be bumped off by assassins called loopers, Rian Johnson's third and most ambitious feature keeps the action popping while sustaining interest in the long arc of a story about a man assigned to kill the 30-years-older version of himself. A lively, high-profile choice to open this year's Toronto International Film Festival, this Sony release co-starring Bruce Willis and Joseph Gordon-Levitt in the same role should chalk up sizable returns in the wake of its Sept. 28 theatrical bow.

Probably the shakiest aspect of Johnson's original screenplay is what it asks the viewer to buy about the future: A mere 62 years from now, in 2074, time travel has become possible, but such a momentous breakthrough is limited to serving as a body-disposal system. Under the prevailing authority, time jumping is strictly outlawed because of its potential for messing with history. A large criminal mob, run by an overlord called The Rainmaker, defiantly uses it but only as a vehicle for assassination, with “loopers” -- disreputable gunmen living in 2044 -- laying in wait for people to execute so no bodies or other evidence can be found in the future.

But the premise is established in nifty fashion; the doomed, hooded with hands bound behind them, suddenly materialize in an empty field, and the looper immediately blows him away with his blunderbuss. One such executioner is Joe (Gordon-Levitt), a retro-looking hipster who drives a very old red Miata and wears ties, “a 20th century affectation” that offends his crankily genial boss, Abe (Jeff Daniels). If he can get out of this racket, he says he'd like to go to France, which earns him further scorn from the older man; “I'm from the future, you should go to China,” he scolds.

Backed by a cynically confessional voice-over track from Joe that is not as self-consciously hardboiled as the commentary Gordon-Levitt read for Johnson in Brick seven years ago, Looper mostly is set in a seedy metropolis that doesn't look all that different from sketchy neighborhoods in some big cities today; there are derelicts, bombed-out buildings, ruined cars and enough other signs of urban ills to suggest that, in Johnson's view, things will just gradually decline over the next three decades.

Joe hangs out in clubs, sees a sexy woman (Piper Perabo) who works in one of them and tries to help a friend and fellow looper, Seth (Paul Dano), who's imminently endangered by a new development that's come down from on high: They're “closing all the loops,” meaning they're sending the “future selves” of all the loopers back to be killed.

Almost immediately, Joe is in the same jam. When, a half-hour into the film, he goes to the field to do his next job, the guy who pops up to be shot is not hooded. Joe's hesitation allows the older man to escape, and it's clear who he is: It's Joe as his older self. And, for his failure to kill him, young Joe is in a pile of trouble with Abe and his “gats,” first-class hired guns.

When the two Joes finally sit down -- across from each other in a diner in the middle of nowhere -- there's no doubt they're working at cross purposes: Young Joe is determined to kill his older self, while old Joe is dead set on tracking down and taking out The Rainmaker, who would be a little kid in 2044, so his late wife won't die at his hands after all.

The biggest problem facing the makers of Looper is how to make the audience believe that the trim, long-faced Gordon-Levitt could somehow change so much in 30 years that he would look like the thicker-built and shorter-nosed Willis. The solution lay in altering the younger actor's appearance, imperceptibly at first, but gradually to morph his dark eyes into Willis' gray-green and to reshape his nose and eyebrows, either with makeup or digitally or perhaps both. At first, the effect is a bit odd, and you can't quite put your finger on what's off; then it feels downright weird to be looking at a version of Gordon-Levitt who is no longer the actor you've known for a few years now.

This is especially noticeable during the film's second half, much of which takes place at young Joe's place of refuge, the isolated home of feisty young farmer and single mom Sara (Emily Blunt), who has an unusually gifted son, Cid (Pierce Gagnon). Even as the temperature is kept at a low simmer, the film's pace deliberately is slowed here to develop some intimacy between these two isolated people and give some screen time to the kid, who pretty obviously will provide the reason for old Joe to eventually head for the farm. The eventual ending is great, the resolution to the tricky time maneuvering very impressively worked out.

Shot mostly in Louisiana, with a bit done in Shanghai, the film looks tightly made on a budget but sacrifices nothing for that; the world depicted looks dirty, dangerous and ramshackle, with a few high-tech touches here and there.

Their physical disparity notwithstanding, Gordon-Levitt and Willis both come across strongly, while Blunt effectively reveals Sara's tough and vulnerable sides. Daniels is particularly amusing as the garrulous old enforcer holding down the future's outpost in the past.